a) Field of the Invention
This invention relates to firing setters (hereinafter simply called xe2x80x9csettersxe2x80x9d) useful upon heat treating or firing (hereinafter collectively called xe2x80x9cfiringxe2x80x9d) ceramic-based electronic device components, led by capacitors, piezoelements and ferrite elements, or high-precision metal-based components produced by injection molding, and also to a process for producing these setters.
b) Description of the Related Art
In recent years, thin ceramic films of various characteristics have been developed, resulting in a wide variety of electronic devices. Illustrative of known ceramic-based electronic device components are ceramic capacitors and multilayer ceramic capacitors (chip capacitors), which make use of dielectric ceramics; piezoelectric transducers and piezoelectric sensors, which make use of piezoceramics; and ceramic actuators (solid-state displacement elements) such as motors and oscillators. They are expected to find a great variety of utility. These electronic devices are formed of ceramic members, for example, in the form of very thin films (tapes or sheets) on the order of micrometers. By using these films singly or stacking them into multilayered structures, electronic devices equipped with various characteristics have been produced. Taking a representative ceramic capacitor as an example, a description will now be made. This capacitor is formed of a member, which consists of barium titanate (BaTiO3) as a principal component and various auxiliary components added to impart characteristics as desired, and electrodes. As an electrode material, palladium, silver, nickel, copper or the like is selectively used depending on the application purpose. Upon production, an organic binder is generally added to a ceramic powder of a desired composition to impart improved processability, followed by forming. In this case, it is necessary to eliminate the organic binder by heat treatment. In some instances, auxiliary components may be calcined beforehand to add them in desired forms to the principal component.
Upon conducting firing or the like of these ceramic-based electronic device components (hereinafter called xe2x80x9celectronic device componentsxe2x80x9d), tray-shaped setters have been used conventionally from the standpoint of productivity to carry electronic device components. Specific examples of such setters include setters obtained by thermally spraying zirconia onto refractories as bases, each of said refractories containing as a primary phase alumina or alumina-silica (mullite) having a particle size distribution in a range of from several micrometers to several millimeters, (see JP 61-24225 A) and setters obtained by coating zirconia onto ceramic surfaces of alumina (JP 3-1090 A).
With a view to making improvements in the productivity of electronic device components and also improvements in the quality of products, there is an outstanding demand in recent years for further improvements in steps, such as firing, in the production process of electronic device components. To meet this demand, it is required to more efficiently eliminate an organic binder, which has been added in a large proportion in the forming step of electronic device components as described above, and moreover, to conduct firing or the like while maintaining the coexisting electrode material in a stable state. As a method for improving the productivity, it may be contemplated, for example, to carry electronic device components as many as possible per unit area of a setter. However, loading of electronic device components in a greater number on any one of the conventionally-applied setter described above makes it difficult to achieve an even temperature distribution or to maintain the uniformity of the atmosphere gas. This leads to a potential problem that the electronic device components may not be produced with uniform quality or may be produced with deteriorated quality. Electronic device components are high-function materials and must be free of such a problem. There is, accordingly, an outstanding desire for the development of a setter which can realize permeability high enough to effectively solve the above-described problem.
Examples of conventionally-known setters which show permeability include those making use of porous ceramics produced by impregnating a porous organic material, which has open cells represented by urethane foam or the like, with ceramics; and setters produced by punching a ceramic sheet or by mechanically forming fine perforations upon forming a plastic mass into a desired shape (see JP 11-79853 A). A further production process of a setter has also been proposed, which comprises forming a shaped body with a spherical organic material of several millimeters in diameter, injecting a ceramic slurry into interstices in the thus-obtained shaped body of the organic material, solidifying the ceramic slurry there, and subjecting the shaped body to firing or the like to remove the organic material, thereby obtaining a porous ceramic body having pores (see JP 63-265880 A).
However, the above-described conventional, permeable setters all require a complex production process, so that they are inferior in productivity and are not economical. According to an investigation by the present inventors, pore of each of permeable setters formed by the conventional processes were of various diameters and were not uniform in diameter, and further, the pores so formed were intricately curved in shape. When firing was conducted, for example, by loading electronic device components on the setter, no smooth permeation of the atmosphere gas was feasible. Even if a conventional permeable setter was provided with many pores, permeation of the atmosphere gas from the outside into the inside of the setter or from the inside to the outside of the setter was still insufficient, thereby making it difficult to conduct firing or the like of electronic device components under even and stable conditions at every area inside the setter.
On the other hand, injection molding has been finding utility in recent years for the production of metal-based components which have heretofore been produced by pressing on a press. Use of a plastic mass obtained by mixing an organic binder in a metal powder material permits injection molding. After obtaining an injection-molded green body of a desired shape, the green body is heat-treated to eliminate the organic binder so that a metal-based precision component of a complex shape is obtained. Metal-based precision components obtained by such a process are now finding increasing utility in various fields. Upon production of such metal-based components, heat treatment of injection-molded green bodies is also conducted to eliminate organic binders as in the above-described production of electronic device components. In the heat treatment, setters are also used from the standpoint of productivity. For example, setters made of a refractory material as a base material, primarily, plate-shaped, economical setters are used.
For the firing or the like upon production of metal-based components by injection molding, there is also an outstanding desire for the development of setters, which can eliminate an organic binder at a low temperature in a short time, especially with a view to making an improvement in productivity. It is possible to achieve an improvement in the productivity of metal-based components available by injection molding provided that setters having high permeability can be obtained at low cost.
As has been described in the above, none of the conventional settersxe2x80x94which have been used in firing or the like upon production of electronic device components or metal-based precision components obtained by injection molding xe2x80x94were able to satisfy at the same time a function to highly eliminate organic substances and volatile components and a function required upon firing electronic device components or the like, that is, a function to achieve a uniform temperature distribution, to permit uniform spreading of atmosphere gas inside the setters and to permit an easy (smooth) transfer of gas to the outside. Needless to say, there is not known any economical process for the easy production of setters, which have such excellent functions as described above, at low cost with excellent productivity.
An object of the present invention is, therefore, to solve the above-described problems of the conventional art and to provide a setter which, when used in a step such as firing performed upon production of electronic device components or the like, can achieve a uniform temperature distribution and even spreading of atmosphere gas inside the setter and an easy transfer of the gas to the outside and can also attain high-efficiency elimination of organic substances and volatile components adhered or remaining on the electronic device components.
Another object of the present invention is to provide a process for easily and economically producing a setter having such excellent functions as described above.
A further object of the present invention is, based on the provision of the excellent setter and its production process as described above, to achieve improvements in the quality of electronic device components or the like and improvements in their productivity and hence to make a contribution to the quality of ceramic-based electronic device components or the like which make use of these components.
The above-described objects can be achieved by the present invention to be described hereinafter. Specifically, the present invention provides a firing setter provided with plural through-holes, said firing setter being useful for a heat treatment or firing step upon production of ceramic-based electronic device components or production of metal-based components obtained by injection molding, wherein the firing setter has an alumina content of at least 70 wt. %, and each of the through-holes is linear with substantially the same inner diameter along a length thereof and has an inner diameter of from 0.3 to 1 mm. The present invention also provides a firing setter as described above, in which the alumina content is 78 to 85 wt. %. This embodiment is suited especially for an application in which the setter is used at elevated temperatures. The present invention also provides a firing setter as described above, in which the alumina content is at least 99 wt. %. This embodiment is suited especially for an application in which a material of the electronic device components is reactive with impurities other than alumina. The present invention also provides a firing setter as described above, in which the firing setter is coated with stabilized zirconia or magnesia at least at a surface thereof where the firing setter may come into contact with the ceramic-based electronic device component or the metal-based components obtained by the injection molding. This embodiment is suited especially for an application in which a material of the electronic device components or the like reacts with a material of the setter.
Further, the present invention also provides a firing setter as described above, in which the firing setter is in a form of a plate provided with plural through-holes therein; a firing setter as described above, in which the firing setter is in a form of a plate provided with plural through-holes therein, and is provided on at least one side thereof with legs which function as spacers; a firing setter as described above, in which the firing setter is in a form of a tray formed of a bottom wall and side walls, and at least one of the bottom wall and side walls is provided with plural through-holes; a firing setter as described above, in which the plural through-holes have an average diameter of from 0.3 to 0.5 mm; and a firing setter as described above, in which the firing setter has a porosity of from 30 to 70 vol. % in a portion where the plural through-holes are arranged.
Moreover, the present invention provides a process for producing any one of the above-described firing setters, which comprises: adding an organic compound to powder, which an alumina content of at least 70 wt. %, to impart plasticity to the powder; forming the thus-plasticized powder into a green body of a desired shape provided with plural through-holes; drying the green body; and firing the thus-dried green body at a temperature of from 1,400 to 1,700xc2x0 C. The present invention also provides a process as described above, which further comprises provisionally firing the dried green body before the firing. The present invention also provides a process as described above, which further comprises machining the thus-fired body into a desired shape subsequent to firing at a temperature of from 1,400 to 1,700xc2x0 C. The present invention also provides a process as described above, in which the organic compound is a polymer having a weight average molecular weight of from 400 to 6,000.
When the setter according to the present invention is used in a heat treatment or firing step of electronic device components or the like, organic substances and volatile components can be eliminated with high efficiency. According to the process of the present invention, the setter can be produced easily and economically. Based on the provision of the excellent setter as described above, the present invention can also achieve stable and high-efficiency production of high-quality electronic device components or the like and at the same time, can make a contribution to the quality and productivity of ceramic-based electronic device products making use of electronic device components or the like.